The measurement of internal body conditions by means of external measurements is the preferred measurement technique for diagnosing disease or monitoring body conditions. The goal for measurement by external measurements is to be as minimally invasive as possible to achieve measurement of the desired internal condition. Many devices have been designed to achieve the above goals through the use of electrical or electrochemical surface measurements.
Another important technique that is becoming increasingly important is the transdermal administration of drugs as a preferred mode of delivery. Transdermal delivery of drugs provides many advantages over other means of administration such as oral or by injection. Advantages include safety, convenience, increased patient compliance, non-interrupted therapy, avoidance of the hepatic first pass effect, and the high degree of control of any administered drug.
Many drugs are not amenable to transdermal delivery due to the well known barrier properties of the skin. Drug molecules that are intended to penetrate into the body through intact skin must first penetrate the stratum corneum and materials therein or on its surface. The drug molecule must then penetrate viable epidermis, the papillary dermis, and then the capillary walls before entering systemic circulation. In its path into the body, each of the mentioned tissues will exhibit different resistances to penetration by the same drug molecule. It is the stratum corneum, however, that presents the greatest barrier to absorption by topical administration or transdermal administration for most molecules. Muscous membranes present similar problems depending on the materials that are desired to penetrate the membrane.